Since the advent of fiber optics, the fiber optical communication infrastructures have become more diverse and sophisticated. The fiber optic applications range from low speed, local area networks to high speed, long distance telecommunication systems. In recent years, the demands for greater bandwidth and lower network costs have resulted in increased component integration and optical devices that offer multiple functions in a single package. For example, optical cross connect switches designed with built-in variable optical attenuators provide power equalization across channels. Photonic integrated circuits route, condition and monitor DWDM wavelengths all within a single package. Popularity of such integrated devices are largely based on the cost savings and performance advantages they offer over individually packaged components. Such integrated devices also simplify coupling and alignment challenges in the optical system and offer lower insertion loss over their individually packaged counterparts.
Optical isolators are used in present day optical fiber networks to block reflected signals from reaching a source laser or LED, and optical isolators are expected to be placed in front or behind a variable optical attenuator in next generation transceiver modules. Optical isolators are typically comprised of a sandwich 1st polarizer, faraday rotator, 2nd polarizer, wherein polarized laser light output parallel to the optical axis of the 1st polarizer passes through the 1st polarizer and is rotated 45 degrees by the faraday rotator prior to passing through the 2nd polarizer which has an optical axis offset 45 degrees from the 1st polarizer to allow the light to pass. In an optical isolator, reflected light passing back through the 2nd polarizer is further rotated by the faraday rotator by another 45 degrees and is absorbed by the 1st polarizer.
As stated previously, a transceiver module often includes a variable optical attenuator connected to the output of the optical isolator to control the laser output signal strength. Variable optical attenuators (VOAS) may be of mechanical or non-mechanical type. Prior art mechanical VOAs include those based on a movable lens which defocuses output light, beam steering mirrors to off steer the center of the light spots away from an output collimators, cantilever arms to assert bends in fiber and shutters to impede the optical transmission path. These methods adjust coupling between two fibers thereby controlling attenuation across an optical path but are known to suffer from reliability issues such as mechanical wear and breakdown. To overcome these issues, non-mechanical VOAS have been introduced during the last several years, including VOAS based on liquid crystal technology.
Liquid crystal is a promising non-mechanical VOA technology with no moving parts. Liquid crystal optical attenuators are generally of a twisted nematic type (TN) comprised of two orthogonal polerizers affixed to the outside sandwich of transparently conductive glass plates each anchored with orthogonal alignment layers. Liquid crystal molecules sealed between the plates of glass homeotropically align with the orthogonal anchor layer and enjoin at the center of the liquid crystal sandwich along a helical twist. Voltage applied across the liquid crystal plates causes the liquid crystal molecules to untwist and realign, in so controllably rotating the polarization of light passing through the cell, creating for variable attenuation of the light source at the output polarizer. However, it is generally known that liquid crystal cells are susceptible to temperature and humidity change, and that high humidity and temperature changes cause decreased optical performance, resulting in high insertion loss and low extinction, two critical measures of a liquid crystal cell's performance.
Recent advances in nano imprint lithography has resulted in the ability cost effectively etch a substrate with sub wavelength optical grating nanostructures, and these nanostructures are known to exhibit unique optical properties as a result of having feature sizes in the hundreds of nanometers to tens of nanometers, below the wavelength of incident light. For example, a glass substrate was recently etched to form a subwavelength optical nanostructure grating on its surface, enabling the glass to perform as polarization filter. In addition, a Faraday rotator substrate has been etched with a similar subwavelength optical nanostructure grating to result in the formation of an integrated isolator.
Given the cost and performance benefits of integration, the assertion that liquid crystal technology is highly compatible with imprint lithography, the potential to generate liquid crystal substrates capable of providing polarization and isolation optical functions, a strong need exists for liquid crystal variable optical attenuator integrated with a discreet polarizer and isolator that also overcomes the aformentioned issues associated with liquid crystal technology.